1. Field of the Invention
This invention relates to an apparatus for forming an image by an electrophotographic or electrostatic recording process in which a developed image (toner image) obtained by being developed on an image carrier by a developer is transferred to a transfer member supported on a transfer member supporting member. More particularly, the present invention can suitably be realized as a color copying machine or a color laser beam printer which forms a multicolor image by repeating transfer of developed images to a transfer member on a transfer member supporting member a number of times in correspondence with the number of color components.
2. Description of the Related Art
Conventionally, a transfer drum 105, such as that shown in FIG. 7, is used, for example, as a transfer device for use in a color electrophotographic copying machine. This transfer drum 105 has parallel flanges 101 and 102 and a connection member 103 which connects the flanges 101 and 102. An opening 104 is defined in the circumferential surface of the transfer drum 105. A transfer member supporting member 106 having a layer of dielectric material such as polyvinylidene fluoride (PVDF) is wrapped around the transfer drum 105 over the opening 104. A corona discharge device 107 is disposed inside the transfer drum 105 to perform a transfer operation by causing corona discharge from the reverse side of the transfer member supporting member 106.
A toothed portion 108 is formed on the flange 102 of the transfer drum 105, as shown in FIG. 7. The toothed portion 108 meshes with a gear (not shown) formed on one of two flanges of an image carrier in the form of, for example, a drum, i.e., a photosensitive drum, to be driven and rotated by this gear.
Hard rubber layers (not shown) are formed to extend outwardly on the outer circumferential surfaces of the flanges. The rubber layers contact the outer circumferential surfaces of the flanges of the photosensitive drum to set the transfer drum and the image carrier in positions in a predetermined relationship. The thickness of a rubber layer is selected so that a gap corresponding to the thickness of a transfer member is formed between the transfer member supporting member and the photosensitive drum.
A pressing member (not shown) is further provided to press the reverse surface of the transfer member supporting member 106 with a small pressure during transfer, so that the transfer member and the photosensitive drum contact at a uniform pressure.
In the thus-constructed transfer drum, however, the area available for supporting the transfer member supporting member 106 at the circumferential surface of the transfer drum 105 is very small and a problem concerning durability of the transfer member supporting member 106 is therefore encountered.
On the other hand, a roller transfer device is known in which a transfer member supporting member 106 comprises a transfer drum having a foamed elastic layer having an outer circumferential surface upon which a dielectric layer is laminated.
In such a known roller transfer device, a suitable pressure for contact between the transfer member and the photosensitive drum is set by a deforming of the elastic foam layer of the transfer drum. Then, the transfer drum having the elastic foam layer is pressed against the photosensitive drum at a constant pressure. A transfer device having a transfer drum having such an elastic foam layer is advantageous because the entire dielectric layer is supported on the elastic foam layer from the reverse side, and the durability of the dielectric layer is higher in comparison with the transfer drum shown in FIG. 7.
Moreover, the roller transfer device requires no corona discharge device and therefore avoids the generation of ozone. These advantages of the roller transfer device have attracted attention.
However, the roller transfer device having the above-described construction also entails drawbacks described below.
That is, if the contact pressure between the image carrier in the form of a photosensitive drum or the like and the transfer drum is excessively large, there is a possibility of a failure to transfer an inner portion of a toner image, i.e., there may be a blank in the transferred image (hereinafter referred to as "inner blank").
Further, if the extent to which the transfer drum is compressed is excessively large, there is a possibility that Q the actual transferred image will be shifted from the position at which the image transferred to the transfer member thereafter is meant to be formed. Such a phenomenon results in a color misalignment in the case of a color image forming apparatus.